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Relativistic Astrophysics

Relativistic astrophysics studies at JPL are concerned with gravitational waves (ripples in spacetime), tests of general relativity, mathematical relativity, and time delay interferometry. Research in this area is very active and is expected to lead to better understandings of gravitational waves, black holes, and the early universe.

Research conducted on relativistic astrophysics at JPL is principally concerned with Solar System tests of General Relativity (Einstein's theory of gravitation), the emerging field of gravitational-wave astronomy, and gamma-ray astrophysics.

JPL is part of the APOLLO collaboration, which does lunar laser ranging using this 3.5-m telescope at Apache Point, NM.

So far, general relativity is in full accord with experiment, yet it cannot be the final word on gravitation (since there is yet no quantum version of the theory), and some string-theory-inspired models predict deviations from general relativity that should be measurable within the Solar System.

JPL has a long history of testing based on Lunar Laser Ranging, and JPL scientists have been developing several concepts for satellite-based tests.

Gravitational waves, which are like ripples in the fabric of spacetime, are emitted by accelerating masses, such as two stars in a binary system. Some gravitational wave sources are fantastically energetic. For example, when two black holes of comparable mass merge, during the time that the merger takes, the energy-per-second radiated by the binary in gravitational waves exceeds by a factor of a thousand the luminosity of all the stars in all the galaxies in the entire observable universe.

However, gravitational waves interact very weakly with any detector (rather like neutrinos), and decades have been spent designing and building detectors of sufficient sensitivity to observe them.

Selected Current Research Projects

Top Image: A schematic diagram of the LISA spacecraft in formation as they orbit around the Sun. The spacecraft are separated from each other by 5 million kilometres and trail behind the Earth at a distance of 50 million kilometres (equivalent to 20 degrees). Copyright: ESA

Bottom Image: A schematic illustration of the orbit carved out by the LISA spacecraft as they revolve with the Earth around the Sun in the course of one year. Copyright: ESA

Laser Interferometer Space Antenna (LISA)

JPL scientists are particularly active in the Laser Interferometry Space Antenna (LISA) project, a planned joint NASA-ESA satellite mission to detect gravitational waves from space in the 0.1-100 milliHz band. LISA will detect gravitational waves from thousands of short-period white-dwarf binaries in our own galaxy, from stellar-mass compact stars falling into giant, from million-solar-mass black holes in distant galactic nuclei, from mergers of giant black holes in galactic nuclei, and perhaps from phase transitions in the very early Universe. In so doing, LISA will open up a new window in astronomy: the window of low-frequency gravitational waves.

Work on LISA has a distinguished heritage at JPL; JPL scientists invented "time delay interferometry," a crucial technology for subtracting laser frequency fluctuations (which noise would otherwise swamp the expected signal). This group of scientists is currently developing LISA data analysis algorithms and software. This is a difficult job, because there is no prior history of such gravitational wave detectors in space. The main technical difficulties arise because 1) most signals will be deeply embedded in noise, and will have to be "dug out" using signal analysis techniques based on matched filtering, and 2) LISA is an "all-sky monitor", so that signals from tens of thousands of resolvable sources are all overlapping in the data stream, and have to be disentangled. This second data analysis problem is called "the cocktail party problem" since it is closely akin to the problem of recording the din from a crowded cocktail party and then trying to disentangle from that recording all of the individual voices. The group has already made great progress in solving these problems, as demonstrated by the Mock LISA Data Challenges: exercises in high synthetic data sets, containing signals whose physical content are secret, are publicly released, and participants are "challenged" to correctly dig out the embedded signals.

JPL researchers are also actively involved in assessing what science can be done with the LISA data, once it is obtained; e.g., what LISA can teach us about mass transfer in close white-dwarf binaries or about the buildup of structure in the early universe.

Laser Interferometer Gravitational-Wave Observatory (LIGO)

LIGO is a facility dedicated to the detection of cosmic gravitational waves and the measurement of these waves for scientific research. It consists of two widely separated ground-based installations within the states of Louisiana and Washington. LIGO has reached its initial design goal of being able to measure strains in spacetime at the 10^{-21} level, and is currently undergoing its first upgrade. After the first or second upgrade, LIGO will almost surely detect gravitational waves from merging binaries of stellar-mass black holes, and JPL researchers are involved in developing the codes that will search for these sources in the data streams.

The science investigations also include LIGO searches for gravitational waves from tiny "hills" on rapidly rotating neutron stars operated in unison as a single observatory. This observatory is available for use by the world scientific community, and is a vital member in a developing global network of gravitational wave observatories. LIGO will view the high-frequency waves from transient phenomena, like supernovae and the final minutes of inspiraling neutron-star binaries. LISA will observe the lower frequency waves from quasi-periodic sources, like compact star binaries long before coalescence, and supermassive black-hole binaries in the final months of coalescence.

Note that this difference in frequency bands makes LISA and LIGO complementary. This range of frequencies is similar to the various types of wavelengths applied in astronomy, such as ultraviolet and infrared.